Adding storm windows can help your older windows perform better, protecting your interior windows from storm damage, serving as a shield and a barrier. More importantly, they provide an additional layer of insulation. Today, a lot of options exist for exterior and interior storm windows, which can be made to fit virtually any window opening and type.

From "Historic Windows & Energy Efficiency" by Sarah Donahue Wolff

Attaching storm windows is another historically accurate way to increase the efficiency of original windows. Historic windows used in combination with storm windows have been shown to have even better insulating properties than double-pane windows, an R value of 1.79 compared to 1.72 for new windows. Exterior storm windows can also provide protection for the historic window and can be designed to minimize their aesthetic impact.

Homeowners have many options when choosing storm windows. The cost of installation depends on many house-specific factors including the region and location of the house, the size and number of windows, their accessibility (first floor? second floor? attic?), the labor required, and the degree to which the windows must be customized to fit a particular space

Interior Storm Windows
Interior storm-window systems differ in material (vinyl, wood) and operation. Some are designed to open and close; others remain attached. Although interior storm windows increase the energy efficiency of the historic window system, they don't protect it from the elements. Moisture can also be trapped between the windows. However, interior storm windows do not obscure the look of the window from the outside, and they are removable.

Aluminum Triple-Track Exterior Storm Windows
Triple-track systems attach to the outside window frame and have two window sash and a screen which move along separate tracks, providing protection while allowing ventilation. They are readily available in a variety of sizes and colors. On the basis of balancing cost, ease and effectiveness, this system is recommended by the National Park Service. Exterior storms can reduce the visual beauty of the historic windows, though some new profiles minimize their impact.

Replacement Traditional Storm/Screen System
New storm/screen systems hang from small hooks on the exterior frame of the window. The system consists of two parts: 1) a storm window with glass in a wood frame and 2) a similar frame with screen instead of glass. The storm windows can be designed to remain closed or to open and close. The wood sash can be painted to blend with the exterior design of the house. This traditional system requires more maintenance than modern storm windows. The storms and screens must be changed manually with the seasons, and homeowners cannot easily switch between storms and screens.

Useful Links

• See this articlefrom This Old House on installing storm windows.